1. Field of the Invention
The present invention relates to an imaging device for use in an optical reading system used in an image forming apparatus or an image reading apparatus such as a copying machine, a facsimile machine, etc. In particular, the present invention relates to an imaging device, for forming an erected image, incorporated in a reading scanner which may be combined with a charge coupled device line sensor with single-magnification, an optical printer head, a self-scanned optical printer head, or other devices.
2. Discussion of the Background
A background imaging device for an optical reading system incorporated in a copying machine, a facsimile machine, or another similar machine is known. For example, an imaging device having a plurality of micro reflective planes aligned in an array is disclosed in Japanese Laid-Open Patent Publication No. 4-130411/1992. The plurality of micro reflective planes have a converging function with respect to a direction parallel to an alignment direction. An angle between the planes is a right angle. Alternatively, when the plane angle at a valley portion of the plurality of reflective planes is a little less than the right angle, the converging function is improved.
The imaging device disclosed in the above-mentioned Patent Application '411 has a cylindrical lens and at least one cylindrical mirror having a generating line parallel to an alignment direction in which reflective planes are aligned periodically, and the generating line of the cylindrical mirror is a straight line included in the converging surface of the cylindrical mirror. In addition, the imaging device is formed to have a generating line of the cylindrical lens parallel to that of the cylindrical mirror. Another imaging device having two cylindrical mirrors for an incident light and an emitted light is also disclosed in the Patent Application '411, and the two cylindrical lenses for the cylindrical mirrors are formed to have respective parallel generating lines relative to the two cylindrical mirrors.
When the above-mentioned imaging device has a plane angle between the reflective planes at a right angle, luminous flux does not converge by the plurality of reflective planes with respect to a perpendicular direction that is perpendicular to the alignment direction, while the luminous flux converges by the cylindrical mirror to form an image. When the imaging device has two cylindrical mirrors for incident light and emitted light, the luminous flux is collected by the first cylindrical mirror with respect to the perpendicular direction. Then, the collected luminous flux is reflected by the reflective planes, and is converged by the next cylindrical mirror to form an image. That is, with respect to the perpendicular direction, the first cylindrical mirror collects the incident luminous flux, and the second cylindrical mirror is subsequently used for the convergence of the collected luminous flux to form an image. On the other hand, as to the luminous flux with respect to a direction parallel with the alignment direction, the luminous flux is imaged by a converging function of the plurality of reflective planes for a diffused luminous flux. That is, light emitted from a point returns approximately to the same point, because the reflected light becomes approximately parallel to the emitted light.
A direction in which the luminous flux converges by the cylindrical mirror is defined as a longitudinal direction, and the direction parallel with the generating line of the cylindrical mirror is defined as a transversal direction that is perpendicular to the longitudinal direction. When the angle between the reflective planes disclosed in the Patent Application '411 is a right angle, the luminous flux to be imaged in the longitudinal direction converges by the cylindrical mirror to form an image. Further, the luminous flux in the transverse direction is converged to form an image by the intrinsic converging function of the plurality of reflective plane pairs.
When the plane angle between the reflective plane pairs at the valley portion is slightly less than 90 degrees, the above-mentioned converging function in the transverse direction is improved so that the reflected light originally emitted from one point returns closer to the original point. Resolution with respect to the transverse direction of the imaging device having the plane angle of 90 degrees is limited by the pitch of the plurality of reflective planes in the alignment direction according to the Patent Application '411. This results because the transverse luminous flux parallel to the generating line of the cylindrical mirrors is converged by the reflective plane pairs. The function of the reflective plane pairs in this case is described such that the diffused transverse light is projected onto the plurality of reflective planes, and the reflected transverse light is parallel to the incident light.
In order to achieve an imaging device for a facsimile machine having, for example, a resolution of the order of 200 dpi (dot per inch) as disclosed in the Patent Application '411, the formation of the plurality of reflective planes is required with a resolution several times higher than that for picture elements. The formation at this resolution may be very difficult.
In addition, since there are edges and valleys at the joining portions of the reflective planes in the plurality of reflective plane pairs, diffuse reflection or light scattering by them is unavoidable. In a practical sense, it is impossible to form an edge with an infinitesimal line width for reducing the effect of the edge, and the edge actually formed has a certain degree of line width. Therefore, an increment of the resolution accompanied by a reduction of the pitch of the plurality of reflective planes increases the fraction of the scattered light by the edge, and the scattered light in a form of flare light affects the image properties, which may deteriorate a contrast of the image.
Furthermore, when the angle between the reflective planes is slightly less than the right angle, although the converging function is improved, reduction of the pitch of the above-mentioned plurality of reflective planes may not be achieved. Therefore, the adoption of a plane angle of slightly less than the right angle may not be effective for reducing the flare light by the diffuse reflection by the edges.
Therefore, the background imaging device still suffers from the above-mentioned problems such as stray light or flare light that may decrease the contrast.